The present invention relates to broadband coaxial cable communications systems interconnecting a plurality of communicating stations and particularly concerns apparatus and methods for adjusting station modem transmit power levels to improve channel throughput, equalize system access by limiting channel capture effect, or compensate for loop losses.
Broadband communications networks, e.g. local area networks, typically comprise a plurality of stations connected to a broadband communication medium such as a broadband coaxial cable through each station's respective RF modem. In a given network, each modem transmits data on an RF carrier having a so-called transmit frequency and receives data on another RF carrier having a so-called receive frequency. The transmission of data between modems in the network is effected by a head-end frequency translator which receives a data signal at the network transmit frequency from a transmitting modem and translates it to the network receive frequency for receipt by the destination modem as well as by the transmitting modem. In this sense, the network forms a closed-loop wherein a transmitted data signal is routed from a transmitting modem to a frequency translator which redirects it back to that modem as well as to a specified destination modem. The retransmitted signal level is linearly proportioned to the level of the original transmitted data signal. Alternatively, the frequency translator may be replaced by a re-modulating head-end wherein the received signal is demodulated and digitally processed and then modulated again and retransmitted in the forward direction.
Modems used in closed-loop communications systems of the type described typically transmit at a fixed signal level, for example about 44 dBmV for four carriers in a 6 MHz bandwidth. This level is selected so that a data signal transmitted from any sending modem will be received by any destination modem at a signal level within the range -15 dBmV to +10 dBmV, with 0 dBmV being the nominal receive level. The carrier-to-noise ratio of data signals received at these levels generally provides adequate system performance. However, signal strength loss between a particular modem and the head-end frequency translator, and between the frequency translator and the modem, may be such that the receive level of a data signal does not fall within the desired range for optimum performance. By way of example, factors such as tap variations, temperature changes, amplifier failures, etc. may all cause undesired signal level variations through the communications medium which may eventually degrade the carrier-to-noise performance of the system. These variations affect larger networks to a greater extent and affect individual modems differently based on their location in the network. In contention-based protocol systems, particularly those of a passive design, signal level including variations caused by different communication path lengths between modems, can result in unequal access to the network because of the well-known capture effect.
Whenever two or more stations begin simultaneous transmission in the channel, the station generating the strongest signal will dominate the access channel and thus will not detect a collision. The weaker stations will detect the collison and execute a back-off algorithm for retransmission on the reverse channel. Thus the strongest station will dominate the weaker and the collision detection aspect of the access protocol will partly fail.
Generation of signal level control signals at the head-end of a two-way cable television facility was taught in U.S. Pat. No. 4,554,579, which is assigned to the assignee of the present invention. The disclosure of that invention was concerned with achieving adequate signal strength levels at the head-end.